I am continually amazed with this observation. Small shapes keep stacking up to become bigger versions of themselves. I don’t know why this is so striking to me. (Why shouldn’t Nature work this way?) It just is. Structure and symmetry can be incredibly captivating! (For a competing view on the beauty of unstructured materials, check out Joerg Heber’s post today on metallic glasses.)

Fortunately for me, it is getting to be that time of year when I’m about to be surrounded by molecular and macro-structure. The weather is turning colder, there’s a perceptible nip in the air, and there is certainly plenty of snow on its way this winter season.

But why do crystals of water look this way? Well, as is the case with table salt, we can learn a lot by looking at water on the molecular level.

As water crystals (snowflakes) form inside of clouds, the water molecules stack up in a hexagonal pattern.

They stack up this way because the oxygen atoms (red balls) like to share their hydrogen atoms (gray bars) with each other (a phenomenon called hydrogen bonding). The stabilizing forces behind hydrogen bonding orient the water molecules into a hexagonal pattern.(Image credit)

They start breaking off from their general hexagonal shape (while retaining their hexagonal symmetry) because the water molecules in the corners have the least amount of stabilizing hydrogen bonds (only a few neighboring water molecules). And they want more.

So, the corner molecules start pulling in more water molecules. (Image credit)

For more on snow crystal science, visit snowcrystals.com, which is run by Ken Libbrecht, another fellow Caltech scientist. (FYI, his snowflake books make fantastic Christmas or Hanukkah gifts, especially for the science geek in your life!)

And, just to prove that there are other materials that can invoke the season check out this image and many like it coming from the research of Song Jin at Wisconsin.

Lead Sulfide pine trees.(Image credit: Journal of the American Chemical Society)

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